The invention pertains to a process for increasing the heat flow density of heat exchangers employing at least one high-velocity gaseous medium, and to a heat exchanger apparatus for undertaking the process.
Because of the decreasing importance of air-cooled automobile engines, there has in recent years been a great decrease in the use of exhaust gas heat for heating the interiors of such vehicles. In vehicles using water-cooled engines, it is simple for the radiator water to be used as a source of energy for interior heating. However, in view of steps being taken for increasing mileage in automobile engineering, there has come to be less and less waste heat from the cooling system of the engine which can be used for this purpose. In view of this, especially in the case of high efficiency engines, it is frequently no longer possible for all the interior and other heating needs of a vehicle to be covered without the use of back-up heating system.
In order to increase the amount of heat transferred by the engine to the engine coolant, a pressure build-up in the exhaust gases has been used. As a result, however, the mileage is decreased, and the temperature of the exhaust gases and the emission of noxious substances is increased. The only source of waste heat which may still be realistically used as a source of energy for such interior heating needs, is the heat of the exhaust gases. If the exhaust gas heat is recovered by a gas-water heat exchanger and thus made part of the heating and cooling system of the vehicle, the mileage and the emission of exhaust gases can be improved because of the increase in the temperature level of the engine.
Some of the earlier shortcomings experienced with exhaust gas heat exchangers, such as cracks caused by thermal stresses and decomposition of the anti-freeze liquid or other engine coolant, have been overcome by providing the exhaust gas-water heat exchanger in a bypass of the exhaust gas system, with the heat exchanger being exposed to exhaust gas only when necessary to get the desired heating effect, and with the water or coolant constantly running through the heat exchanger to keep it at a more or less constant temperature.
The main shortcoming of such heat exchangers has, however, been the dependency of the usable waste heat of the exhaust gas on the engine power output, such power output varying over a wide power ratio of about 200 to 1 in the case of diesel and gasoline engines, for example. Because the amount of usuable heat of the engine coolant is dependent on the engine power output as well, the need for a stepped-up heating effect is highest in those cases in which there is the least usable waste heat of the exhaust gas, thus necessitating large-area and heavy heat exchangers that take up much space. This tendency goes against current attempts to down-size and reduce the weight of a vehicle and the decrease, going hand in hand therewith, in the space on hand. Therefore, since the amount and temperature of the exhaust gas is relatively low at low engine output, it is desirable to increase the exhaust gas heat flow density by other measures.
One way of increasing the exhaust gas heat flow density is to increase the velocity of the exhaust gas contacting the heat exchanger surface. By increasing the velocity of the exhaust gas flow, the heat transmission coefficient (k-value) is increased, which especially in the case of gases is a function of the flow velocity. Also, the price, overall size, and weight of the heat exchanger are greatly dependent on the heat transmission coefficient.
However, there are economic limits to the degree to which the exhaust gas velocity may be increased because such increases in velocity generally mean that the flow cross-sections are decreased. In order to obtain the higher pressure difference necessary for the higher flow velocity, it is necessary for gas blowers and blower drives to be made larger and more complex. Furthermore, the operating costs are greatly increased by the higher use of energy.
In motor vehicles, such as private passenger automobiles, the manner in which the flow velocity in such heat exchangers is increased is especially important because of undesired effects on costs, weight and overall size of the vehicle and because of the power needed to operate the blower. It is first necessary to make certain that the required blower power is supplied to the blower drive, thus making a more powerful dynamo necessary and in turn being responsible for an undesired increase in weight and an undesired increase in the overall size of the vehicle. The most important factor in connection with the operation of a high pressure blower is, however, the especially poor efficiency typically obtained in connection with producing the necessary driving power. It is necessary for the pressure energy, which is needed for increasing the component speeds through the system, to be increased many times, and furthermore the efficiency in a motor vehicle is very poor because a number of components or systems with poor efficiency are interrelated or interconnected with a multiplicative effect on the overall vehicle efficiency. The chain is made up of the links, that is to say: higher pressure energy times worse efficiency of the centrifugal blower times poor efficiency of the blower drive times poor efficiency of the dynamo times poor efficiency of the vehicle engine. Such effects thus have the multiplicative effect of reducing the overall efficiency of the vehicle.
Because a high fluid velocity has previously been found to be uneconomical, automobile heat exchangers have frequently been designed for the lowest possible pressure loss, that is to say the lowest possible flow velocity. For this reason, the normal heat transmission coefficients (k-values) in automobile engineering are generally 20 to 50 Watt/sq.m/degree Kelvin.
It is known for exhaust gas heat exchangers of motor vehicles with piston engines to be connected directly to the exhaust system of the engine so that a separate blower is unnecessary. Because such heat exchangers have generally been designed for low fluid pressures and velocities, such heat exchangers have been relatively conventionally designed. However, as will be made clear in the present specification, this is unnecessary because it has been found that the shortcomings noted above may be overcome if heat exchangers capable of a high heat flow density are provided.
The radiator of an automobile engine is a heat exchanger, whose previous weight and overall size have stood in the way of attempts at saving fuel. In order to save fuel, a decrease in vehicle weight and aerodynamic resistance is desired. For this reason, there is a need for smaller radiators with a lower weight. Meeting this need is possible by increasing the heat flow density of the heat exchanger used as the radiator.
For this reason, one purpose of the invention is that of economically increasing the heat flow density of heat exchangers having at least one gaseous medium flowing through them at a high velocity. This is accomplished without costly measures being necessary for producing high pressures and/or high velocities through the heat exchanger, without unduly adding to the technical complexity of such heat exchangers, and furthermore without increasing the amount of energy used by, and the weight and overall size of, the vehicle. Decreasing the weight and overall size of the vehicle is especially desirable in automobile engineering.
In the case of the process and apparatus described herein, the above purpose may be effected if the energy for acceleration of the gaseous medium is taken from the exhaust gases of an internal combustion engine. From this it may be seen that an energy supply can be provided with only low equipment costs and with low energy consumption because the energy needed may be supplied in the form of otherwise wasted energy.
Preferably, the invention may be used in heat exchangers for motor vehicle interior heating. A useful development of the invention may be produced by increasing the energy content of the exhaust gas by causing a pressure build-up in the exhaust gas. As discussed above, there is a further need for heat energy especially when the engine is running with a small load. In contrast to the operation of a conventional heat exchanger, another purpose of the present invention is that at a low load on the engine high heat flow densities may be produced, while at medium engine load, the lowest possible heat flow densities are produced.
Because of the above-mentioned pressure build-up, the exhaust gas temperature is increased, thus desirably increasing heat transfer at the exhaust gas heat exchanger; while on the other hand the coolant water temperature is also increased, thus increasing the heat transfer for normal vehicle heating. If the pressure build-up is produced within, or at the inlet of, the heat exchanger, high exhaust gas velocities along the heat exchanger surfaces will be produced even at low engine output, and low velocities are produced by reducing the pressure build-up at high engine powers. The compression power needed for producing higher exhaust gas velocities, is produced by the engine itself through the gas pressure build-up. Thus, due to the exhaust gas pressure build-up at a low engine load, the density and the temperature of the exhaust gases are increased and the transmission of heat from the engine to the engine coolant is stepped up.
As a further useful development of the invention, the exhaust gases of the internal combustion engine can be fed into a pump apparatus (used for accelerating a gaseous medium) in order to cause the exhaust gases to give up their kinetic energy. According to one form of the invention in which the heat exchanger for cooling an engine coolant has a gaseous medium flowing therethrough, the pump apparatus can be turned on or put into the circuit whenever the coolant temperature exceeds a predetermined value.
Exhaust gas heat exchangers of conventional design have the shortcoming that their efficiency goes up with increasing engine loads at which they are less frequently needed, and their efficiency goes down with decreasing engine loads at which they are more frequently needed.
The use of waste energy from engine exhaust gases for increasing the heat flow density in heat exchangers for vehicle interior and other heating, and the use of heat exchangers for cooling an internal combustion engine, results in an increased heat availability for heating purposes at low engine loads, while in this condition of operation an increase in the heat flow density at the radiator is not necessary. On the other hand, at a high engine load there is a desire to get a better cooling effect by increasing the heat flow density in the radiator, while in this condition of operation there is no need for further heating energy. Between these two conditions of operation there are intermediate conditions in which there is no need for a stepped-up cooling or a stepped-up heating effect.
As part of another useful development in the invention, a heat exchanger system for use in the invention may have a heat exchanger placed in the flow path of the exhaust gas and a pressure build-up unit that may be selectively operated, or selectively put into and taken out of the circuit. As part of one especially useful form of the invention, the duct for the gaseous medium of the heat exchanger is of such a size that between the inlet and the outlet for the gaseous medium, a pressure difference of approximately 0.01 and 0.5 bar and a flow velocity of approximately 50 to 200 m/sec. are obtained.
Such a heat exchanger design, which in its size or dimensions is different than normally-used heat exchangers, makes it possible for a heat exchanger system to be well matched to conditions of use in motor vehicles, because it is especially designed for the middle part of the engine load range. In such load ranges, the transmission of heat to the coolant of the engine is not adequate for the heating needs, while on the other hand the waste heat in the exhaust gas is adequate to meet the eating needs, so that the use of the pressure build-up unit is generally not necessary in this high load condition of operation. At least one form of the heat exchangers disclosed herein, with a flow velocity greater than that of normal heat exchangers, creates a very intensive effect so that it is herein referred to as an "intensive heat exchanger". The increase in exhaust gas pressure necessary for operation of the intensive heat exchangers does not make any changes necessary in the exhaust gas and engine systems. The energy for the compression of the exhaust gases is supplied by the engine itself. Because of the compression of the exhaust gases there is an increase in fuel consumption. However, such increase is small, because the compressor efficiency of the piston engine is much better than that of a centrifugal compressor, and because there is no need for an electric motor for driving a blower and thus there is a lower load on the generator or dynamo and the engine losses are changed into useful heat, but for a loss by radiation of about 10%. It may be seen from this that the mileage of the engine only decreases a very small amount that is negligible within the limits of measuring accuracy. Changes in the design of the exhaust and engine system are not therefore deemed necessary.
At low engine loads, an intensive heat exchanger is of nearly no effect because of the low temperature of the exhaust gases and because of the decrease in the heat transmission coefficient (k-value). The decrease in the k-value is because the exhaust gas volumes at low engine loads are much lower than at medium loads so that the flow velocity through the heat exchanger goes down and, because of this, the k-value decreases. Because of the low resistance to flow through the heat exchanger, however, there is generally no decrease in mileage at such engine loads. At high engine loads, however, the exhaust gas volumes and temperatures are much greater than at medium loads. For this reason, a large amount of heat is available in the exhaust gas system, and the heat exchanger is very efficient because of the high flow velocities. Such efficiency is further increased by friction effects and by the higher density of the exhaust gases in connection with the exhaust gas pressure build-up resulting from the higher velocity. Furthermore, in conditions of operation with a high engine load, enough waste heat will be present in the coolant from the engine for heating the vehicle, and the function of the exhaust gas heat exchanger is thus not necessary in this case. Therefore, because of the great amount of heat at the intensive heat exchanger at high engine loads, the heat exchanger must be bypassed by the exhaust gases. As a signal for the bypass condition, it is possible to make use of the throttle of the carburetor (in the case of a carburetor engine) or the position of the control rod of an injection pump (in the case of a fuel injection engine), or the temperature of the exhaust gases.
On the other hand, because an intensive heat exchanger is of normally nearly no effect at a low engine load, the pressure build-up unit has to be used as part of the invention. Although, in such an intensive heat exchanger according to one form of the invention, the pressure build-up unit is necessary, such a heat exchanger has a lower overall size and a lower weight (with the effect of increasing the mileage and cutting down equipment costs) than normally-used heat exchangers because of the more narrow flow cross-sections used in producing the higher flow velocities.
The pressure build-up unit may be placed at any position along the flow path of the exhaust gases. However, in especially useful forms of the heat exchanger system of the invention, the pressure build-up unit is disposed at the heat exchanger in the form of a nozzle system, for example, on the side of the gas inlet into the heat exchanger. The nozzles direct the exhaust gas flow against the heat exchanger surface so that the velocity of the exhaust gas along the heat exchanger walls is increased, and thus higher heat flow densities, higher gas densities, and higher exhaust gas temperatures are obtained.
Another possible means for producing the pressure build-up is by making the exhaust gas flow duct in the heat exchanger of such a size that, between the inlet and outlet, there is a pressure difference of approximately 0.1 and 5 bar and a flow velocity equal to approximately 100 to 500 m/sec. In such a design or size of heat exchanger, the flow velocities are so high that a high velocity gradient is created in the flowing gas, thus producing the necessary friction for increasing the temperature of the gas and producing better heat transmission. Furthermore, because of the high flow velocities, relatively high pressure build-ups are produced so that there is a marked increase in the exhaust gas temperature from the engine, and the exhaust gas density is also greatly increased, thus increasing the heat transmission coefficient. The heat transmission coefficient increases generally as a function of the 0.8th power of the exhaust gas density. Because of internal friction, this design of heat exchanger is referred to herein as "a friction heat exchanger". The factors determining the heat flow density in a friction heat exchanger, the factors together taking effect are the high temperature difference caused by the pressure build-up effect, the high heat transmission coefficient resulting from the higher density, the high heat transmission coefficient resulting from the high flow velocity along the heat exchanger surface, and the high temperature resulting from internal friction of the gases or from friction of the gas on the heat exchanger surfaces.
Additional useful developments of the invention will become apparent from the description below and from the appended claims.
A more detailed account of the invention will now be given by way of some working examples of the heat exchanger system of the invention, as illustrated schematically in the drawings.